Pervaporation assembly

ABSTRACT

The invention is a pervaporation process and pervaporation equipment, using a series of membrane modules, and including inter-module reheating of the feed solution under treatment. The inter-module heating is achieved within the tube or vessel in which the modules are housed, thereby avoiding the need to repeatedly extract the feed solution from the membrane module train.

This application is a divisional of U.S. application Ser. No.11/651,303, which was filed on Jan. 9, 2007 and is now U.S. Pat. No.7,758,754, the disclosure of which is hereby incorporated by referencein its entirety.

This invention was made in part with Government support under SBIR awardnumber DE-FG02-05ER84244 awarded by the Department of Energy. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to pervaporation. In particular, the inventionrelates to pervaporation processes in which multiple membrane modulesare contained in a single housing or vessel.

BACKGROUND OF THE INVENTION

In pervaporation, a multi-component liquid stream is passed across amembrane that preferentially permeates one or more of the components. Asthe feed liquid flows across the membrane surface, the preferentiallypermeated components pass through the membrane and are removed as apermeate vapor.

Transport through the membrane is induced by maintaining a vaporpressure on the permeate side of the membrane that is lower than thevapor pressure of the feed liquid. The vapor pressure difference isusually achieved by maintaining the feed liquid at a higher temperaturethan that of the permeate stream. The latent heat of evaporation of thepermeate components must be supplied to the feed liquid for the feedtemperature to be maintained and for the pervaporation process tocontinue.

In a typical separation, such as that of an alcohol from water, the feedcools about 5° C. for every 1% of the feed that permeates the membrane.In industrial pervaporation processes, an average of about 5% of thefeed permeates the membrane per module. The corresponding temperaturedrop is thus about 25° C. Other separations may involve greater orlesser temperature drops.

The temperature drop is reversed by withdrawing the feed solution andrunning it through individual heaters or heat exchange cycles betweeneach module. This is commonly referred to as inter-stage heating.

To accommodate these requirements for inter-module or inter-stagereheating, pervaporation systems must include numerous pipes, flanges,valves and other fittings to enable the feed solution to pass in and outof the vessel housing the modules. This makes the systems complex toengineer, cumbersome to build, and costly, and limits the industrialutility of pervaporation.

In consequence, although commercial pervaporation systems have beenavailable for more than twenty years, few practical applications for aprocess that is otherwise attractive have been realized.

Co-owned U.S. Pat. Nos. 7,404,843 and 7,510,594, and co-owned,co-pending U.S. patent application Ser. No. 11/484,547, disclose gasseparation equipment in which multiple membrane modules in multipletubes are contained in a single housing. These three patent applicationsare incorporated herein by reference in their entireties.

SUMMARY OF THE INVENTION

The invention is a pervaporation process and pervaporation equipment forseparating a component from a liquid mixture, the mixture typicallycontaining water and at least one organic component, or being a mixtureof at least two organic components.

The separation is carried out by running a feed stream of the liquidmixture across a separation membrane under pervaporation conditions. Bypervaporation conditions, we mean that the vapor pressure of thecomponent that it is desired to separate into the permeate stream ismaintained at a lower level on the permeate side than on the feed side,and the pressure on the permeate side is such that the permeate is inthe gas phase as it emerges from the membrane. The process results,therefore, in a permeate vapor stream enriched in the desired componentand a residue liquid stream depleted in that component.

In a first aspect, the process is carried out using multiple membranemodules or elements arranged in series within a single tube, so that theresidue stream exiting the first module in the series forms the feed tothe second module, and so on, until the final or product residue streamis withdrawn from the last module in the series.

To maintain adequate transmembrane flux, the feed solution undertreatment is heated within the tube as it passes from one module to thenext. This interstage heating or reheating is achieved by blocking thestraight flow path from the residue end of one module to the feed end ofthe next, and by heating the outside surface of the tube. Instead ofpassing directly to the inlet of the next module, the feed is directedin a flow path in the annular space between the inside wall or surfaceof the tube and the outer casing or surface of the membrane module thatit has just exited. By forcing the stream to flow at least partiallyback along the outside of the module, it is brought into heat exchangingcontact with the inside surface of the tube.

In a basic embodiment, the pervaporation process of the inventionincludes the following steps:

(a) passing a feed solution to be treated through a series of multiplemembrane modules, each membrane module having an outer longitudinalsurface, a feed end and a residue end, the membrane modules being housedin a single tube having an inside and an outside surface, to separatethe feed solution under pervaporation conditions into a residue streamand a permeate stream; and

(b) achieving an inter-module reheating of the feed solution as itpasses along the series by:

-   -   (i) flowing the feed solution exiting the residue end of a        membrane module in the series at least partially back toward the        feed end of that membrane module in a reheating zone defined by        the outer longitudinal surface of that membrane module and the        inside surface of the tube before the feed solution is permitted        to enter the feed end of the next membrane module in the series;        and    -   (ii) heating the outside surface of the tube.

The process divides the feed stream into a treated residue stream and apermeate stream, either or both of which may be desired products of theprocess. For example, if the feed solution is a dilute solution ofethanol in water, the process of the invention may be used to form amore concentrated ethanol product as the permeate stream. Likewise, ifthe feed solution is ethanol containing just a few percent of water, theprocess of the invention may be used to dehydrate the ethanol, forming apurified ethanol product as the residue stream.

The membrane modules or elements are housed in a tube. The tube servesto house and support the membrane elements and provide a directed fluidflow. In addition, the tube conducts heat to warm the feed solution asit passes along the train of modules, and may provide apressure-withstanding function if the pressure conditions under whichthe separation process is carried out are substantially different fromthe pressure outside the tube.

The outside of the tube may be heated in any appropriate manner.Preferably, low grade steam is used if available.

The membrane used to perform the separation may be any type of membranecapable of performing an appropriate separation under pervaporationconditions. Suitable membranes include polymeric membranes, inorganicmembranes, such as ceramic membranes, and membranes containing inorganicparticles embedded in a polymeric matrix. For example, if the feedsolution is to be dehydrated, a hydrophilic membrane, such as apolyvinyl alcohol membrane, may be used. If the feed solution is amixture of olefins and paraffins, a hydrophobic membrane, such as afluorinated polyimide membrane, may be used.

The membranes and modules may take any convenient cylindrical form, suchas flat sheets wound into spiral-wound modules, potted hollow fibers ortubular membranes that will fit into the tube so as to leave an annularspace between the outer longitudinal surface of a membrane module andthe inside surface of the tube. The configuration of the process andapparatus of the invention is not suitable for plate-and-frame modules,as these are usually assembled in stacks, not housed in tubes orcylindrical pressure vessels.

The series includes at least two modules, and will typically includethree, four, five or six modules mounted end to end in the tube. Themodules are connected as described above such that a feed stream undertreatment may enter the feed end of the first module, flow through themodules in turn and exit as a final residue stream from the residue endof the last module. The modules are also connected by a permeate pipe orpipes, through which the collected permeate stream from the series canflow.

The driving force for transmembrane permeation is the difference betweenthe vapor pressure of the feed liquid and the vapor pressure on thepermeate side. This pressure difference is generated at least in part byoperating with the feed liquid at above ambient temperature, usuallyabove 30° C., and typically in the range 30-120° C. Optionally, thepermeate side may also be maintained under vacuum to increase thedriving force.

To heat the feed solution as it passes along the chain of modules, thefeed solution is prevented from flowing in a straight line immediatelyfrom the residue end of one module to the feed of the next. Instead, thefeed solution exiting the residue end of a module is directed at leastpartially back along the outside of the module it has just exited, intoa reheating space or zone between the outer longitudinal surface of thatmodule and the inside surface of the tube. The reheated residue solutionis then directed out of the reheating space to the feed inlet end of thenext module.

Any flow-blocking and flow-directing means that achieves this flow pathcan be used within the scope of the invention.

A preferred means is an adapted end cap. The caps are mounted on theresidue ends of the modules and engage in fluid-sealing manner againstthe inside surface of the tube. Liquid leaving the module is directed bysuitably configured channels within the end cap, so that it exits intothe reheating space associated with that module, and must flow throughor across that space before passing through another channel in the endcap that leads toward the feed end of the next module. To improve flowdistribution within the reheating zone, straight or curved baffles, finsor ribs may extend along the reheating zone.

Another example of a suitable flow-blocking and flow-directing means isa specially adapted flow-directing plate positioned between one moduleand the next in the series.

The process of the invention provides an improved technique for carryingout any pervaporation operation that requires the use of multiplemodules in series. The new process avoids the need to extract thesolution under treatment from the membrane module train to run itrepeatedly through external heaters or heat exchangers. Not only doesthis provide better heat integration, but the large numbers of pipes,valves, flanges and fittings associated with repeated removal andreintroduction are eliminated. As a commercial pervaporation train maycontain as many as ten pervaporation-reheating steps, the savings inengineering complexity and cost is substantial.

In a second aspect, the invention provides for separation of a liquidmixture according to the principles described above, but in this casethere are multiple series of modules, mounted in multiple tubes, and thetubes themselves are contained within a single outer housing, assemblyor vessel. In this case, the savings in complexity and cost are evenmore marked.

In this aspect, the process of the invention includes the followingsteps:

(a) providing a vessel in which multiple tubes are mounted in parallel,each tube having an inside and an outside surface and containing aseries of multiple membrane modules, each membrane module having anouter longitudinal surface, a feed end and a residue end;

(b) passing a feed solution to be treated through the membrane modulesto separate the feed solution under pervaporation conditions into aresidue stream and a permeate stream;

(c) achieving an inter-module reheating of the feed solution as itpasses along the series of modules by:

-   -   (i) flowing the feed solution exiting the residue end of a        membrane module in the series at least partially back toward the        feed end of that membrane module in a reheating zone defined by        the outer longitudinal surface of that membrane module and the        inside surface of the tube before the feed solution is permitted        to enter the feed end of the next membrane module in the series;        and    -   (ii) heating the outside surfaces of the tubes by flowing a        heating fluid through the vessel.

In this case, the preferences for the layout within each tube ofmembrane modules and flow-blocking and flow-directing elements aresimilar to those for the single-tube process.

In a third aspect, the invention is the pervaporation equipment, systemor apparatus adapted to carry out the pervaporation separation process.In this aspect, the invention includes the following elements:

(a) a series of multiple membrane modules, each membrane module havingan outer longitudinal surface, a feed end and a residue end, andincluding a permeate pipe protruding from the membrane module, themembrane modules having their permeate pipes connected in an end-to-endmanner;

(b) a tube containing the membrane modules, the tube comprising at leastone removable head and a shell having an inside surface and an outsidesurface, the tube being provided with a feed inlet port and a residueoutlet port and adapted so that a permeate stream flowing through thepermeate pipes may be withdrawn from the tube;

(c) an annular seal for each membrane module, positioned so as toprovide a fluid-tight seal between the outer longitudinal surface of themodule and the inside surface of the tube;

(d) flow-blocking and flow-directing means positioned in the tube so asto block immediate flow of a fluid from the residue end of a membranemodule to the feed end of the next membrane module in the series;

(e) an annular reheating zone between the outer longitudinal surface ofa membrane module and the inside surface of the tube;

the flow-blocking and flow-directing means being adapted to directresidue fluid into and out of the reheating zone; and

(f) means for heating the outside surface of the tube.

The flow-blocking and flow-directing means is preferably an adapted endcap with a flow-directing channel and an outlet bore, as describedabove, or a flow-directing plate positioned between sequential membranemodules in the series.

The means for heating the outside surface of the tube is preferably acasing around the tube, through which a heating fluid can be passed.

The equipment may use multiple tubes, with a series of membrane moduleswithin each tube. In this case, a single vessel is used to house thetubes, and the means of heating the outside surface of the tubes ispreferably to circulate a heating fluid, such as steam, through theinterior of the vessel outside the tubes.

If a multi-tube vessel is used, a representative and convenient numberof tubes is seven tubes, and there are preferably at least threemembrane modules in each tube.

It is to be understood that the above summary and the following detaileddescription are intended to explain and illustrate the invention withoutrestricting its scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing the flow pattern in apervaporation process of the invention.

FIG. 2 is a schematic drawing showing, in longitudinal cross-section,the layout of modules and end caps in a pervaporation process of theinvention in which the membrane modules are contained within a singletube.

FIG. 3 is a schematic drawing showing a representative configuration ofthe end cap at the residue end of a module in longitudinalcross-section.

FIG. 4 is a schematic drawing showing a representative cross-section ofan end cap such as that of FIG. 3 as it faces the residue end of amodule.

FIG. 5 is a schematic drawing showing a view of an end cap such as thatof FIG. 3 as it faces the feed end of the next module in series.

FIG. 6 is a schematic drawing showing an alternative arrangement of theflow channels in a residue end cap.

FIG. 7 is a schematic drawing showing, in longitudinal cross-section,the layout of modules and end caps in a pervaporation process of theinvention in which the membrane modules are contained within multipletubes in a single housing.

FIG. 8 is a schematic cross-sectional drawing showing placement of 7tubes in an assembly such as that of FIG. 7.

FIG. 9 is a schematic drawing showing the positioning of two ribs todivide the reheating space.

FIG. 10 is a schematic drawing showing, in longitudinal cross-section,an alternative arrangement for blocking and redirecting fluid flow atthe residue end of a module.

FIG. 11 shows the blocking arrangement of FIG. 10 in radialcross-section.

FIG. 12 is a plot showing temperature profiles along two membranemodules in a one-tube assembly similar to that of FIG. 2.

FIG. 13 is a plot comparing temperature profiles along two membranemodules in a one-tube assembly similar to that of FIG. 2 at differenttemperature differentials between the heating fluid and the feedsolution.

DETAILED DESCRIPTION OF THE INVENTION

All percentages herein are by weight unless otherwise stated.

The terms membrane module and membrane element are used interchangeablyherein.

The terms tube, housing and vessel are used interchangeably as theyrelate to apparatus in which all of the membrane modules are containedin a single tube.

The terms assembly, housing and vessel are used interchangeably as theyrelate to apparatus in which all the membrane modules are contained inmultiple tubes within a single housing.

The terms reheating space, reheating area and reheating zone have thesame meaning and are used interchangeably herein.

The invention is a process for separating a component from a solution bypervaporation, and the equipment or apparatus to carry out suchseparation.

Any solution that may be treated by pervaporation may be treated by theprocess of the invention. Most commonly, the liquid to be treated willbe a solution of one or more organic components in water, or of water inan organic solvent or solvent mixture, but solutions containing onlyorganic or only inorganic components may also be treated. Separation ofaromatics from paraffins in an oil refinery, removal of organic sulfurcompounds from hydrocarbon mixtures, dehydration of bioethanol, recoveryof ethanol from fermentation broth, and removal of volatile organiccompounds (VOCs) from wastewater are typical representative examples ofseparations in which the process of the invention can be used toadvantage.

The separation is carried out by running a feed stream of the liquidmixture across a separation membrane under pervaporation conditions. Bypervaporation conditions, we mean that the vapor pressure of thecomponent that it is desired to separate into the permeate stream ismaintained at a lower level on the permeate side than on the feed side,and the pressure on the permeate side is such that the permeate is inthe gas phase as it emerges from the membrane. The process results,therefore, in a permeate vapor stream enriched in the desired componentor components and a residue liquid stream depleted in that component orcomponents.

In a first aspect, the process is carried out using multiple membranemodules or elements arranged in series within a single tube.

A significant feature of the invention is that the feed solution neednot be withdrawn from the tube for reheating. Instead, the feed solutionis heated within the tube as it passes from module to module.

FIG. 1 is a schematic drawing showing the general flow pattern in arepresentative pervaporation process according to this aspect. Referringto FIG. 1, the process is carried out in a tube or housing, 101, using aseries of four membrane modules, 102, 103, 104 and 105.

The feed solution to be treated enters the process through line 106.Instead of passing directly from module 102 to module 103, the residueof the feed solution from module 102 flows in the reheating pathindicated as 107, which takes it back along the outside of the modulewithin the housing, and thence into the second module 103 in the series.In similar fashion, the residue from module 103 flows in reheating path,108, from module 104 flows in reheating path, 109, and from module 105flows in reheating path, 110. The treated solution leaves the housingand the process through line 111.

It will be apparent from FIG. 1 that an equivalent process could becarried out by reversing the direction of the flow pattern, so that thefeed solution to be treated enters as stream 111, and the treatedresidue stream exits as stream 106.

FIG. 1 shows the residue from the last module in the series beingreheated before leaving the process. This is often desirable, as well asbeing the simplest way to configure the process, in that the same typesof fittings may be used for the last module in series as for the othermodules. Optionally, however, the last residue stream can be withdrawndirectly from the last module without directing it along a reheatingpath.

The invention is now described in greater details in its variousaspects. In the following description, the inventors have not dwelt atlength on the choice, manufacture and combination of conventionalcomponents of the equipment used to carry out their invention. Thedesign and assembly of such is well known in the chemical engineeringart, and is familiar to the engineer or readily available from standardindustry literature.

It will be appreciated by those of skill in the art that the figures arevery simple schematic diagrams, intended to make clear the key aspectsof the equipment and processes of the invention, and that equipment may,and often will, include additional components of a standard type, suchas seals, O-rings, connectors, pipes, feed end caps, flanges, bolts, andother fittings to join and seal components in fluid-tight manner asnecessary.

FIG. 2 is a schematic drawing showing, in longitudinal cross-section, arepresentative, non-limiting layout of modules and end caps in apervaporation process of the invention in which the membrane modules arecontained within a single tube. Referring to this figure, a housing,pressure vessel or tube, 201, contains a series of membrane modules, inthis case represented by three modules, 204, of which only one islabeled to avoid excessive numbers of lead lines over the drawing.Likewise, to assist clarity, other components or elements associatedwith each module, such as the residue end cap, seals and feed end, areidentified only once in the drawing.

The housing takes the form of a cylindrical shell, having inside, 226,and outside, 227, surfaces and equipped with two removable heads, 202and 203. In the drawing, the heads are shown as flanged, and assumed tobe connected to the shell by bolts (not shown), although any convenientmeans to connect the heads to the shell is intended to be within thescope of this embodiment.

In the embodiment shown in FIG. 2, both heads are drawn as removable.This arrangement provides the greatest flexibility for assembly,maintenance and repair, because the membrane elements can be loaded orremoved from either end.

Alternatively, the design can be simplified by permanently welding end203 to the body of the vessel or manufacturing as a unitary part of thebody of the vessel. The modules must then be loaded or unloaded from oneend only, but the manufacturing cost of the vessel may be reduced.

The tube or housing may be made of any convenient material. Housings areusually made of metal, conforming to appropriate codes for the operatingconditions to which they are to be exposed. Pervaporation processes arenot usually operated at feed pressures substantially different fromatmospheric, although they may be operated at high temperatures, above100° C. In the case that the feed is introduced at ambient pressure, and40° C., for example, a housing made from a plastic may suffice, so longas the material has adequate thermal conductivity. In the case that thefeed is under high hydraulic pressure, or very hot, a stainless orcarbon steel housing, for example, may be needed. In general, we preferto use metal housings.

A feed port, 217, and a residue port, 218, are positioned near the endsof the housing. One or both of the end plates or heads is fitted with,or adapted to accept, permeate collection pipe, 209, through whichtreated permeate is removed from the processing train. Alternatively, aflanged permeate port to which the permeate pipes are connected could beprovided.

The membrane modules or elements, 204, each having an outer longitudinalsurface, 225, are arranged in line along the tube. To illustrate thearrangement, three modules in series are shown in FIG. 2. As a generalguideline, we prefer to use at least two modules and no more than aboutsix, although our processes may be carried out with any number ofmodules in the tube.

The modules may contain any type of membranes capable of separating thefeed solution by pervaporation. They may be inorganic or polymeric, andmay be packaged in any manner that enables them to fit in series withinthe housing. For example, inorganic membranes may be in tubular form,with the selective membrane on the inner or outer surface. This type ofmodule is sometimes used when the separation membranes themselves areinorganic, or are supported on an inorganic support membrane, forexample.

If the membranes are polymeric, they may be prepared as flat sheets andpackaged as spiral-wound modules, or as hollow fibers and packaged aspotted hollow-fiber modules, for example.

These forms are well known in the art and are described copiously in theliterature. For simplicity, therefore, the details of module placementand connection, and fluid flow around and within the modules, aredescribed below as they relate to polymeric membranes packaged asspiral-wound modules. Those of skill in the art will appreciate thatsimilar arrangements can be used for other types of modules, subjectonly to minor, straightforward modifications as need be.

A spiral-wound module comprises one or more membrane envelopes ofspacers and membrane wound around a perforated central permeatecollection pipe, 206. Typically, the pipe protrudes from the module atboth ends, as shown in the figure. The pipe may be made of any suitablematerial, such as plastic or metal.

When the module is in use for pervaporation, feed liquid enters at feedend, 223, and passes longitudinally down the module across the membraneenvelope. A portion of the feed permeates as vapor into the membraneenvelope, where it spirals towards the center, is drawn through theperforations into the permeate collection pipe and exits through the endof the pipe. The residue of the feed solution exits the module at theresidue end, 224.

The modules are connected end-to-end, meaning that permeate gas leavingone module can flow into the permeate pipe of the next module. This canbe achieved by having one long continuously formed pipe around whichmultiple membrane modules are wrapped. More preferably, however, thepermeate pipes of the individual modules are separate pipes joined bygas-tight connectors or couplings, 207. If permeate is to be withdrawnfrom one end only, the line of pipes is sealed at the other end by endcap, 208.

The modules are sealed against the tube walls by annular seals, 205, toprevent feed solution bypassing the module, and to separate the residuesolution that has exited a module from the feed solution entering thatmodule.

At the residue end of the modules are end caps, 210, typically made ofsteel or plastic. A larger view of a non-limiting, representative endcap configuration is shown in FIG. 3. Referring to this figure, tube,301, contains a module, 302, seated in end cap, 304. The end cap istypically secured in fluid-tight connection against the tube by anO-ring or other seal, not shown. The end cap is adapted so that permeatecollection pipe, 303, may fit through it. This may be accomplished invarious ways, such as by forming the end cap as two semicircular pieces,as discussed more below, or by providing a suitably sized aperture inthe end cap and using a fluid-tight seal to prevent residue solutionleakage through the aperture.

The module is held securely in the end cap, typically by gluing, so thata small residue space, into which residue liquid leaving the membranescan pass, is provided. One or more fluid-directing channels, 306, areprovided in the cap, so that residue solution exiting the module isdirected as shown by the arrow into the annular reheating space, area orzone, 308.

An outlet bore or channel, 307, directs residue fluid as shown by thearrow from the reheating space to the feed space, 309, of the nextmodule.

A radial cross-section of end cap 304, taken along line A-A′ looking inthe direction of the arrows, is shown in FIG. 4. The end cap has ashallow cup shape, with a rim portion, 401, and a base portion, 402.

At the center of the base portion is a circular opening, 403, sized toaccommodate the module permeate pipe. Channel, 405, is bored rightthrough the thickness of the rim and base, and corresponds to outlet 307in FIG. 3. The bore is shown as cylindrical, but could be any desiredshape. The figure shows one bore; multiple bores could also be used.

Opening, 404, represents the open end of channel 306 in FIG. 3. This isalso shown as cylindrical, but could be any other desired shape, such asa slit. Again, one or multiple channels may be used. To facilitate themachining of this channel, the cap is preferably manufactured in twohalves and fixed together along line B-B′.

A radial cross-section taken along line A-A′ and viewed from the otherside, as it would face the feed end of the next module when in place inthe tube, is shown in FIG. 5. Base portion, 501, is perforated bypermeate pipe opening, 502, and bore, 503, through which the residueemerges as feed for the next module.

Returning to FIG. 2, each module is surrounded by an annular reheatingspace, area or zone, 211, (corresponding to space 308 in FIG. 3),defined by the outer surface of the module, the inside surface of thetube, seal 205, and the residue end cap. Fluid to be reheated entersthis zone through aperture, 212 (the outlet from channel 306 in FIG. 3),and leaves for the next module in series through bore, 213(corresponding to element 307 in FIG. 3).

An outer casing, 214, is fitted to the outside of the tube, and equippedwith ports, 215 and 216, through which a heating fluid, for examplesteam or hot oil, may be passed. In the alternative, any means ofheating the outside of the tube may be substituted.

The pervaporation process of the invention is now described in arepresentative way as it is carried out using the system of FIG. 2.

The feed solution to be treated enters as shown by dashed arrow, 219,through the feed port. A driving force for transmembrane permeation isprovided in the normal manner for pervaporation by maintaining the vaporpressure of the feed liquid higher than the vapor pressure on thepermeate side.

Although any technique may be used to achieve this pressure difference,the commonest and simplest way is to heat the feed solution prior tointroducing it into the equipment. The temperature to which the feed isheated may be chosen by the skilled artisan in consideration of thespecific circumstances of the operation. For aqueous feed solutions, atemperature between about 30° C. and 100° C. is generally used. Forsolutions of higher boiling point, or that are held under elevatedpressure, higher temperatures, such as 120° C. or more, are possible andmay be preferred in some circumstances to increase flux.

In pervaporation, the permeate side of the membranes is held at pressureand temperature conditions that result in a vapor-phase permeate. Thelow pressure on the permeate side may be achieved in the normal manner,such as by simply cooling and condensing the permeate as it is withdrawnfrom the system or by using a vacuum pump to draw a partial vacuum.

The feed solution enters the first module, where it is separatedaccording to pervaporation principles into a residue solution (that is,the residue of the feed solution that remains on the feed side of themembranes and that exits the residue end of the module), and a permeatevapor, each having a different composition from the feed solution.Representative examples include:

(a) a feed solution comprising about 10% ethanol in water, separatedinto a permeate vapor containing 40% ethanol and a residue solutioncontaining 2 or 3% ethanol, using silicone rubber membranes in themodules;

(b) a feed solution of raw gasoline in a refinery comprising a total of30% toluene, benzene and other aromatics, separated into a residuegasoline stream containing below 25% total aromatics and a permeatevapor containing 70% aromatics, using a fluorinated dioxole membrane;

(c) a feed solution of 5% water in acetic acid, dehydrated to a 0.5%water residue solution and a permeate vapor containing 60% water vapor,using a polyvinyl alcohol (PVA) membrane.

As the separation occurs, the latent heat required to evaporate thepermeating components is supplied from the feed liquid, so the residuesolution leaving the module is significantly cooler, such as 5° C., 10°C. or more cooler, depending on how much permeate vapor is produced.

The residue solution exits the module through aperture 212 and flowsinto and across the reheating space, as indicated generally by dashedarrow, 220, to become the feed solution for the next module.

A heating fluid is passed through the outer casing, as indicated bydashed arrows, 228 and 229, and flows in contact with the outsidesurface of the tube. The residue of the feed solution that is flowing inthe reheating space is reheated by heat-exchange with the heating fluidacross the tube wall.

The separation and subsequent reheating steps are repeated along thetrain of modules, and the final treated residue stream is withdrawn asindicated by dashed arrow 221.

The permeate vapor from each module is collected in the permeate pipes,206 and withdrawn from the system as indicated by dashed arrow, 222.

Those of skill in the art will appreciate that the blocking of immediateflow of fluid from the residue end of one module to the feed end of thenext module, and the directing of residue fluid into, across and out ofthe reheating space, could be achieved by any flow-blocking andflow-directing means suitably positioned within the tube between, or atthe ends of, the modules, so long as that means is functionallyequivalent to the apparatus elements shown in the figures so far.

As an example, one simple equivalent is shown in FIG. 6, in which aresidue end cap, 601, in shown in similar view to FIG. 4. As in FIG. 4,bore, 606, passes through the full thickness of rim, 602, and base, 603,to carry residue fluid from the reheating space of one module to thefeed inlet of the next. Aperture, 604, is provided for the permeatepipe.

Instead of channel opening 404, the cap of FIG. 6 has a set of notches,605, in one side of the rim, cut down as far as the base portion. Whenthe module is inserted into its cap, the notches enable fluid to flowfrom the residue space to the reheating space. The cap may be made astwo pieces, for example one half having the notches and the other theresidue outlet bore, and assembled by joining the pieces along lineC-C′.

An optional enhancement that promotes circulation of the fluid to theend of the reheating space nearer to the feed end of the module is shownin FIG. 9. Referring to this figure, FIG. 9 a shows a module and residueend cap as viewed with the end cap to the left of the figure; FIG. 9 bshows the module as viewed with the residue end cap to the right. Inboth cases, the size of the cap is exaggerated for clarity.

Referring to FIG. 9 a, module 901 is fitted with residue end cap, 902,through which protrudes permeate pipe, 903. Bore, 904, provides passagefor the residue fluid out of the reheating space of one module to thefeed space and feed end of the next. Two baffles or fins, 905 and 906,are positioned diametrically opposite to one another. The baffles areattached to the end cap, from which they extend within and partiallyalong the reheating zone.

Each baffle has a width about the same as the radial thickness of therim of the end cap. This may be seen more clearly in FIG. 9 b, whichsimply shows, without any other details, the positioning of baffle 905with respect to cap 902 and module 901. When the module with its end capis inserted into the tube, not shown, the baffle will sit against theouter longitudinal wall of the module and the inside wall of the tube.This fit need not be fluid-tight, as the purpose of the baffle is simplyto direct fluid flow to some extent.

When the module is in use in the process of the invention, residue fluidfrom the module will be constrained by the baffles to flow in a path asgenerally indicated by dashed line, 907 in FIG. 9 a. That is, theresidue fluid will exit the end cap into the reheating space, where itwill be directed along the length of the module by the baffles until itcan pass to the other side of the baffles and be drawn out through bore,904.

The baffles are most conveniently attached to the end cap, although itwill be apparent that they could be attached to the module or, lesspreferably, even to the inside of the tube.

FIG. 9 shows one pair of baffles or fins. More pairs, in conjunctionwith multiple fluid-directing channels in the end cap, could optionallybe used to facilitate distribution of the residue solution in thereheating space.

FIG. 9 also shows the baffles as straight, although curved baffles couldoptionally be used to direct the residue fluid in a curved path, such asa helical path, instead.

An example of a different way to achieve blocking of straight-line flowof fluid from the residue end of one module to the feed end of the nextis shown in FIG. 10. In this case, fluid exits the residue end of themodule in conventional manner, but the flow-blocking and flow-directingmeans takes the form of an adapted flow-directing plate that is addedbetween modules.

Referring to FIG. 10, tube, 1001, contains a series of modules, 1002, ofwhich only one representative module is shown in full. The modules aresealed against the tube walls by annular seals, 1003, and are equippedwith permeate pipes, 1004, which are connected together by couplings orconnectors, 1005.

Instead of the adapted, flow-directing end caps shown in FIGS. 2-6, theapparatus is provided with flow-directing plates, 1006, through whichthe permeate pipes can pass as shown. In some regards, these platesresemble tube sheets, in that they can hold the permeate pipes in place.

More importantly, however, the plates block the straight-line flow offluid from the residue end of one module to the feed end of the next.The plates are perforated by hollow tubes or ducts, 1007, that extendfrom the plate, so that they lie along the reheating space, 1009, over aportion of the length of the module.

The positioning of the hollow ducts is more easily seen in radialcross-section, as in FIG. 11. In this view, flow-directing plate, 1101,has a solid face, 1102, an aperture, 1103, through which a permeate pipecan pass, and a set of hollow ducts or tubes, 1104, positioned aroundthe periphery of face 1102 outside the perimeter, 1105, of the module.

As with the baffles of FIG. 9, the ducts are shown as straight tubes,but could be curved.

To carry out the process of the invention, a feed liquid for separationis passed through the train of modules, generally as described abovewith respect to FIG. 2. In this case, liquid exiting the residue end ofthe module is constrained by the flow-directing plate to flow into thereheating space 1009 according to the flow path indicated generally bydashed line, 1008. After it has passed across the reheating space,liquid enters the hollow tubes, and flows through them to exit into thefeed space, 1010, of the next module.

In a second aspect, the invention uses multiple series of modules,mounted in multiple tubes, the tubes being mounted in a single assembly,vessel or housing. The invention in this aspect has certain features incommon with co-owned U.S. Pat. Nos. 7,404,843 and 7,510,594, andco-owned, co-pending U.S. patent application Ser. No. 11/484,547, whichdescribe the uses and advantages of multi-tube housings for gasseparation. Additional information regarding disposition ofcorresponding elements of the apparatus, operation, benefits, and so onmay be found in these patent applications.

FIG. 7 is a schematic drawing showing, in longitudinal cross-section, arepresentative, non-limiting layout of modules and end caps to carry outa preferred pervaporation process of the invention using such aconfiguration. The details of the arrangement of modules, permeate pipesand end caps within a tube that were described with respect to FIG. 2are applicable within the tubes to this embodiment also.

Referring to FIG. 7, the assembly includes a vessel, 701, containing aplurality of tubes, 708. Three tubes, of which only the top one islabeled to avoid long lead lines over other parts of the drawing, arevisible in the longitudinal sectional view. The figure represents anassembly with 7 tubes in total, shown in radial view in FIG. 8,discussed below. The ends of the tubes are open to allow fluid flow intoand out of the tubes.

The housing has feed and permeate ends, 702. In the drawing, the feedand permeate ends are shown as removable flanged heads, connected to thebody of the shell by bolts (not shown). However, any convenient means ofconnection of the ends is intended to be within the scope of thisembodiment, and in some variants, discussed below, only one end need beremovable.

The housing has five ports for admitting or removing fluids. A feedport, 705, is positioned near the feed end, and residue port, 706, andpermeate port, 707, are positioned near the permeate end. Ports, 703 and704, give access to the interior, 716, of the housing outside the tubes.A heating fluid, such as steam or hot oil, can be circulated through thehousing by passing the hot fluid in at port 703 and withdrawing fromport 704 (or vice versa).

Each tube, 708, contains membrane elements or modules, 709. For clarity,the membrane module(s) are drawn in full in the top tube, and indicatedonly at the ends and by the dashed portions in the other tubes. Fourmembrane elements are shown, although any convenient number could beused.

Permeate pipe, 712, represents the total length of permeate pipes andconnectors, and is usually configured with a separate permeate pipe foreach membrane element, as shown in FIG. 2, for example. The modules aresealed against the tube walls by annular seals, 710, at the feed end. Atthe residue end, each module has a residue end cap, 711. The end capsmay be configured in any manner, such as those discussed already andshown in FIGS. 3, 4, 5 and 6, that enables feed solution under treatmentto exit as a residue stream into the reheating spaces, 717, and to passthence to the feed end of the next module, in similar manner to thatshown in FIG. 2.

Alternatively, any other method of blocking and directing fluid flowfrom one module to the next, such as by using the flow-directing platesshown in FIGS. 10 and 11, may be used.

A feed-end tube sheet, 713, is welded or otherwise mounted in thehousing towards the feed end. This tube sheet supports the tubes inspaced-apart relationship with each other.

At the other end of the housing, two tube sheets are provided. Tubesheet, 714, supports the modules and directs residue fluid from theindividual tubes to the residue port, 706. Tube sheet, 715, allowspassage of the permeate pipes and directs the combined permeate streamsfrom the individual permeate pipes to the permeate port, 707.

In the embodiment shown in FIG. 7, both tube sheets 713 and 714 haveopenings that correspond in width to the tube diameter, and both headsare drawn as removable. This arrangement provides the greatestflexibility for assembly or maintenance.

If tube sheet 715 is welded or otherwise permanently fixed in thehousing, as will generally be the case, then it must also have apertureslarge enough to pass the modules through, if the ability to load orunload the modules from the permeate end is needed. This may beaccomplished by providing large apertures, but sealing the annular spacearound the permeate pipe with an end-plate, as shown in FIG. 2 of Ser.No. 11/050,995.

If the modules cannot be passed through tube sheet 715, then thepermeate end of the housing may be permanently welded or formed as aunitary part with the shell of the housing.

FIG. 8 shows a radial cross-sectional layout diagram of a preferredassembly, such as that of FIG. 7, containing seven tubes. Thecross-section shows the placement of the tubes within the housing orassembly, as viewed looking in at the feed end of the housing.

Referring to FIG. 8, housing or assembly, 801, is equipped with flange,802, having bolt holes, 803, for attaching the feed end or head. Thefeed-end tube sheet, 804, has openings, 805, for supporting the tubes.From the feed end of module, 807, protrudes permeate pipe, 808.

Seals, 806, correspond to annular seals, 710, in FIG. 7, and hold themodules in fluid-tight relationship against the tube walls. Feed port,810, and steam or other heating fluid port, 809, are visible.

Within the limits of engineering practicability, the housing may containany number of tubes. For example, another ring of tubes could be addedoutside the ring of six in FIG. 8, for a total of 19 tubes.

To carry out the process of the invention using a multi-tube housing ofthe type shown in FIG. 7, steam or other heating fluid is circulated inthe housing outside the tubes. The feed solution to be treated isintroduced through port 705, enters the individual tubes and is treatedin the manner described above with reference to FIG. 2.

Heat exchange takes place across the tube walls between the heatingfluid flowing in interior space 716 and the residue fluid flowing in thereheating spaces, 717. The treated residue-stream exits each tube andflows out of the assembly through port 706. The permeate vapor flowsalong tubes 712 and out of the assembly through port 707.

In a third aspect, the invention is the pervaporation equipment, systemor apparatus used to carry out the pervaporation separation processesdescribed above. In the case that only one tube of modules is used, theapparatus is as described and shown by FIG. 2 and equivalents. In thecase that multiple tubes in a single assembly are used, the equipment isas described and shown by FIG. 7 and equivalents.

The invention is now illustrated in further detail by specific examples.These examples are intended to further clarify the invention, and arenot intended to limit the scope in any way.

EXAMPLES Example 1

A bench-scale version of the apparatus of FIG. 2 was built. The tubecontained two 4-inch diameter modules, containing ethanol-selectivemembranes. The housing was fitted with thermocouples so that thetemperature of the feed solution could be measured as it flowed throughthe apparatus. In this way, the actual feed solution temperature dropduring a pervaporation experiment could be compared to the theoreticaltemperature drop that would have occurred in the absence of reheating.

A series of pervaporation tests using a feed solution containing about10 wt % ethanol in water was performed. In each test, 25 gallons ofethanol/water feed solution were loaded into a feed tank and circulatedthrough the system. The permeate pressure was maintained at about 50torr. Hot oil was circulated as heating fluid. At steady-state,temperature readings were obtained from the thermocouples at variouslocations along the module tube.

Measurements of the quantity and composition of the permeate were usedto calculate the permeance and selectivity of the membrane at variousconditions.

The permeance calculations were used to predict the theoretical heatloss that would have occurred if there were no reheating. The resultsfor a set of experiments in which the feed solution was initially at 58°C., the oil temperature was 69° C., the feed flow rate was 1 gpm, andabout 0.8 wt % of the feed solution passed through the membranes in eachmodule are shown in FIG. 12.

If the separation had been conducted under conditions in which noexternal heating was supplied, the temperature of the final residuesolution would have dropped to about 49° C., that is, about 9° C. lowerthan the feed solution temperature, as shown by the straight line in thefigure.

Since transmembrane flux depends on the vapor pressure of the permeatingcomponents, and vapor pressure depends on temperature, a temperaturedrop of 9° C. corresponds to a significant drop in transmembrane flux(up to about 35% loss of flux) for this separation.

Using the apparatus and process in accordance with the invention, theoriginal feed temperature was restored by reheating the residue of thefeed solution from the first module before the solution entered thesecond module. In this experiment, a difference of 11° C. between theheating fluid and initial feed fluid temperatures was sufficient toreheat the residue solution to the desired operating temperature (andhence to maintain transmembrane flux), as shown by the jagged line.

The experiments showed that the invention provides an effectivepervaporation process, without needing to reheat the feed solutionoutside the tube.

Example 2

Another set of experiments was carried out following the same procedureas described for Example 1, except that the heating fluid was at 90° C.instead of 69° C., and the initial feed solution temperature was 72° C.,not 58° C.

The results of this set of experiments are compared with those ofExample 1 in FIG. 13.

As can be seen, at a feed temperature of 58° C., the temperature dropalong a single module was only about 3-4° C., whereas at a feedtemperature of 72° C., the temperature drop was about doubled, at 8-10°C. per module. At higher feed temperature, the transmembrane flux ishigher, so the amount of permeate to be evaporated is greater, and theheat extracted from the feed solution to do this is greater, resultingin a greater temperature drop.

The greater temperature drop means that to maintain the entering feedtemperature for each module at 72° C., a temperature difference of 18°C. between the feed solution and the heating fluid was required. Incontrast, at a feed temperature of 58° C., with lower flux and lowertemperature drop, a temperature difference of only 11° C. was required.

We claim:
 1. A pervaporation assembly, comprising: (a) a series ofmultiple membrane modules, each membrane module having an outerlongitudinal surface, a feed end and a residue end, and including apermeate pipe protruding from the membrane module, the membrane moduleshaving their permeate pipes connected in an end-to-end manner; (b) atube containing the membrane modules, the tube comprising at least oneremovable head and a shell having an inside surface and an outsidesurface, the tube being provided with a feed inlet port and a residueoutlet port and adapted so that a permeate stream flowing through thepermeate pipes may be withdrawn from the tube; (c) an annular seal foreach membrane module, positioned so as to provide a fluid-tight sealbetween the outer longitudinal surface of the module and the insidesurface of the tube; (d) flow-blocking and flow-directing meanspositioned in the tube so as to block immediate flow of a fluid from theresidue end of a membrane module to the feed end of the next membranemodule in the series; (e) an annular reheating zone between the outerlongitudinal surface of a membrane module and the inside surface of thetube; the flow-blocking and flow-directing means being adapted to directresidue fluid into and out of the reheating zone; and (f) means forheating the outside surface of the tube.
 2. The pervaporation assemblyof claim 1, wherein the flow-blocking and flow-directing means comprisesan end cap mounted on the residue end, the end cap including aflow-directing channel to provide fluid communication from the residueend to the reheating zone and an outlet bore to provide fluidcommunication from the reheating zone to the feed end of the nextmembrane module in the series.
 3. The pervaporation assembly of claim 2,further comprising multiple baffles extending from the end cap into andpartially along the reheating zone.
 4. The pervaporation assembly ofclaim 1, wherein the flow-blocking and flow-directing means comprises aflow-directing plate positioned between sequential membrane modules inthe series.
 5. The pervaporation assembly of claim 1, wherein the meansfor heating the outside surface comprises a casing fitted to the outsideof the tube, and equipped with ports through which a heating fluid maybe passed.
 6. The pervaporation assembly of claim 1, comprising multipleseries of membrane modules, contained in multiple tubes, and furthercomprising a vessel in which the tubes are mounted, the vessel having aninterior and being equipped with ports through which a heating fluid maybe circulated in the interior outside the tubes, this heating fluidbeing the means for heating the outside surface of the tubes.
 7. Thepervaporation assembly of claim 6, wherein the vessel contains seventubes.
 8. The pervaporation assembly of claim 6, wherein the seriescomprise at least three membrane modules.